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This problem trains for: SAT-I, AMC-8, GMAT.

Three buses serve a loop-shaped line with a total length of 12 miles. Assuming that the buses are equally spaced and run at the same average speed of 48 miles per hour, what is the probability for a passenger who shows up at a random time at a bus station to get on a bus during the first five minutes of waiting?

Each bus needs 5 minutes to transit from one station to the next. This is because there are 4 miles between two adjacent station. Since the bus travels 48 miles in 60 minutes, it will travel 4 miles in 5 minutes (distance-time are directly proportional quantities).

Therefore, for a passenger showing up at a random time at a station, the probability of getting on a bus in the first 5 minutes of waiting is 1 (this is a certain event).